1. Field of the Invention
The present invention relates to a liquid state detecting apparatus for detecting the concentration of a liquid contained in a liquid container.
2. Description of the Related Art
Recently, a catalyst for selective catalytic reduction (SCR) of NOx is sometimes used in an exhaust gas cleaner for reducing nitrogen oxide (NOx) exhausted, for example, from a diesel automobile into a harmless gas, and an aqueous urea solution is used as the reducing agent. It is known that the use of an aqueous urea solution having a urea concentration of 32.5% by weight allows for efficient reduction. However, the urea concentration of an aqueous urea solution contained in a urea-water tank carried by an automobile may change as time passes. Further, a foreign liquid or water may be introduced into a urea-water tank by mistake. Under these circumstances, in order to allow control over the urea concentration of the aqueous urea solution, the concentration is detected by mounting a concentration sensor for detecting the urea concentration in the urea-water tank.
It is known that the thermal conductivity of an aqueous urea solution varies depending on its concentration. Under these circumstances, a concentration sensor having two temperature sensing bodies (sensor elements) having a resistance which changes with the temperature disposed thereon in parallel is mounted in the urea-water tank, and one of the temperature sensing bodies is heated by energizing the same. The conduction of heat to the other temperature sensing body is affected by the concentration of the aqueous urea solution, and the urea concentration of the aqueous urea solution can therefore be detected based on the difference between the measured resistance values of the two bodies (for example, see Japanese Patent No. 3686672). When the detected urea concentration is outside a certain range, abnormalities can be detected including a determination that a foreign liquid or water is included in the urea-water tank and a determination that no aqueous urea solution is present.
When a diesel automobile is driven, the aqueous urea solution contained in the urea-water tank may be shaken or agitated by the vibration of the vehicle body. In the case of the concentration sensor of Japanese Patent No. 3686672, urea concentration detected by the same may be read as a value that is greatly different from what it should be. This can occur when the resistance values of the sensor elements become disproportionate to the urea concentration as a result of the flow (flow generated as a result of shaking or agitation) of the aqueous urea solution, which in turn can cause local irregularities in the concentration of the aqueous urea solution or remove heat from the sensor elements. For this reason, according to Japanese Patent No. 3686672, whether or not the aqueous urea solution is in a static state is determined based on the driving state of the diesel automobile (specifically, the vehicle speed). If the liquid is not in a static state, it is assumed that there is a possibility of erroneous detection, and the determination of an abnormality in concentration is made using a different weighting from that used in a static state.
3. Problems to be Solved by the Invention
However, according to Japanese Patent No. 3686672, the speed of the diesel automobile is used to indirectly indicate whether an aqueous urea solution in a urea-water tank is in a static state. Since this indirect indication may differ from the actual agitation or flow mode of the aqueous urea solution, a problem has arisen in that an abnormality in concentration may not be accurately determined.
The invention was made to solve the above-described problems, and it is an object of the invention to provide a liquid state detecting apparatus which can determine a static state of a liquid using an output that is separately obtained in a process of detecting the concentration of the liquid by energizing a heating resistor.